1. Field of the Invention
The present invention relates to a VR-type (variable reluctance type) resolver which is mounted on a motor to detect the angular position and rotational speed of the motor and, in more particular, to such VR-type resolver in which the number of teeth formed on a rotor or the number of the stator pole is adjusted to thereby improve the angular accuracy thereof.
The present application is based on Japanese Patent Applications No. Hei. 10-141327, 10-184453 and 11-114810, which are incorporated herein by reference.
2. Description of the Related Art
A VR-type resolver is a device which is structured such that reluctance in an air gap between the iron core of a rotor thereof and the magnetic pole of a stator thereof is caused to vary depending on the position of the rotor and, each time the rotor iron core is rotated, the fundamental wave component of variations in the reluctance provides an N cycle, whereby the variations in the reluctance can be detected and thus the rotational angular position or rotating speed of a motor can be detected by the thus detected reluctance variations. The VR-type resolver has a wide range of uses; for example, it can be used as rotation detect means which feedback controls the rotation angle of a stepping motor with a closed loop to thereby allow the stepping motor to be positioned with high accuracy, while the stepping motor is often used as a direct drive motor (DD motor) in various machines and instruments, including rotational drive units such as a robot, a delivery machine and the like, index tables for use in an automatic assembling machine, a measuring instrument and the like, working indexes for use in an NC machine tool, an exclusively designed machine and the like. Here, FIG. 21 shows a conventional VR-type resolver when it is used in a DD motor, which is disclosed in Japanese Patent Publication No. Hei. 3-150041. The DD motor shown in FIG. 21, which is a VR-type stepping motor M, is structured such that, outside a stator 2 which is composed of an electromagnet fixed to a motor casing 1, there is disposed a rotor 3 composed of an iron core including a large number of teeth formed on and projected from the inner periphery thereof in such a manner that the rotor 3 is opposed to the stator 2. On the rotor 3 of the stepping motor M, there is fixedly mounted, together with a motor output shaft 4 (which is rotatably supported by the motor casing 1 through bearings 5 and 6), a rotor 7 of a resolver R serving as a rotation detector in such a manner that the resolver rotor 7 can be rotated synchronously with the motor rotor 3. On the other hand, a stator 8 of the resolver R is disposed opposed to the rotor 7 (the rotor shown in FIG. 21 is a rotor of an outer type which includes teeth on the inner peripheral surface of the rotor) of the resolver 8 with an air gap between them, while the stator 8 is fixedly mounted on the motor casing 1.
In the above-mentioned conventional resolver R, the magnetic poles of the stator 8 of the resolver R respectively include similar pole teeth to those of the stator 2 of the motor M, while the coils CL of the resolver R are wound around their respective magnetic poles of the stator 8. Also, the rotor 7 of the resolver R includes a large number of teeth which are provided on and projected from the inner peripheral surface thereof, while the number of teeth of the rotor 7 may be set variously, such as, 100, 120, 160 and the like; in any case, the teeth number of the rotor 7 is set equal to the teeth number (pitch) of the rotor 3 of the motor M.
By the way, the DD motor shown in FIG. 21 is a closed type actuator which can be used in the ultra-high vacuum atmosphere. In particular, in the shown DD motor, in order to be able to prevent the vacuum atmosphere from being contaminated by gas released from the coils CL of the stator 2 of the motor M and the stator 8 of the resolver R, in an air gap which exists not only between the motor stator 2 and motor rotor 3 but also between the resolver rotor 7 and resolver stator 8, there is disposed a separation wall 9 formed of non-magnetic metal such as stainless steel or the like; that is, the stator side is covered airtight by the separation wall 9 and is there by isolated from the rotor side. In other words, if a VR resolver is mounted on the thus structured DD motor and the detect signal of the VR resolver is fed back, then the rotational speed and rotational angle (position) of the actuator can be controlled with accuracy.
However, in the above-mentioned conventional VR type resolver, because the number of the rotor teeth is set large such as 100, 120, 160 and the like in order to be able to obtain a high resolving power, and also because the teeth peak shape of the lamination of the resolver is square similarly to that of the lamination of the VR motor, working of the resolver required becomes severer as the diameter of the resolver becomes smaller, which not only makes it easy to produce a teeth pitch error but also makes it difficult to control the air gap in assembling. Accumulated teeth pitch errors worsen the whole periphery accumulation accuracy of the motor rotation position detection, while the uneven air gap gives rise to increases in the stationary angle error, vibration, and noise of the resolver.
Also, when the resolver is mounted on the DD motor, since a motor control device gives a rotation instruction while it is always reading a position detect signal fed back from the resolver to thereby confirm the current position of the motor rotor, if the detected position of the motor rotor by the resolver has an error, then the current position of the motor rotor is different from the rotation instruction by the motor control device, which gives rise to generation of vibrations in the resolver. In this case, as the teeth number of the resolver rotor is larger, a vibration frequency generated becomes higher and thus larger vibrations can be generated. For example, when a motor having the teeth number of 120 is rotating at a rate of 1 rps, if the rotor teeth number of a resolver mounted on the present motor is also 120, that is, equal to the rotor teeth number of the motor, then there is generated a vibration frequency of 120 Hz/rps.
In a DD motor such as the above-mentioned DD motor for vacuum (closed type actuator) in which the separation wall is disposed in the air gap between the respective stators and rotors of the motor and resolver, since the conventional air gap is very small, the thickness of the partition wall must be set very small, which incurs a possibility that the partition wall can be expanded when the motor is used under the ultra-high vacuum condition. Therefore, in the conventional resolver, there is room for improvement.
Further, as another type of position detector, for example, there is known a VR-type synchronous resolver which is disclosed in Japanese Patent Publication No. Hei. 7-44813. The VR-type synchronous resolver, as shown in FIG. 22, is a resolver of an inner rotor type which includes an annular-shaped stator 10 and an annular-shaped rotor 20 in such a manner that the rotor 20 is combined with the inside of the stator 10; and, the stator 10 includes a plurality of (for example, 18) magnetic poles 12 which are formed on the inner peripheral surface thereof and arranged at regular intervals in the circumferential direction of the stator 10 and respectively have a plurality of pole teeth 11 in their respective leading end portions with coils 13 wound around the respective magnetic poles 12, while the stator 10 is fixedly supported. The rotor 20 includes a large number of teeth 21 (for example, 150 teeth) which are formed on the outer peripheral surface thereof and arranged in the circumferential direction of the rotor 20 at the same pitch as the pole teeth 11 of the stator 10 in such a manner that they are opposed to the magnetic poles 12, while the rotor 20 is disposed concentrically with the stator 10 and is so supported as to be rotatable with respect to the stator 10. And, the coil windings 13 wound around the respective magnetic poles 12 of the stator 10, as shown in FIG. 23, are respectively connected to the A phase, B phase and C phase of a 3-phase alternating current in which the three phases are shifted electrically by 120.degree. from one another, so that they are allowed to correspond to the phases of the coils of the motor. In order that the phases of the two electrically mutually adjoining magnetic poles 12, 12 of the resolver stator 10 can have an electric angle of 120.degree. with respect to each other, the pole teeth 11 of the resolver stator 10 are respectively shifted from one another at a pitch in the range of integer times to 1/3 of the pitch of the teeth 21 of the resolver rotor 20.
Now, assuming that the resolver rotor 20 rotates synchronously with a motor rotor (not shown), reluctance in an air gap between the teeth 21 of the resolver rotor 20 and the magnetic poles 12 of the resolver stator 10 varies depending on the change of the position of the resolver rotor 20, and a current depending on the variations in the reluctance is thereby allowed to flow in the coil windings 13 of the resolver stator 10. By detecting the current as a modulation signal, the rotational angle position or rotational speed of the resolver rotor 20 can be detected. If the resolver rotor 20 rotates by an amount equivalent to one tooth thereof, then there is detected on the resolver stator 10 side the following modulation signal which is equivalent to an electric angle of 360.degree.: ##EQU1##
For example, when the teeth number of the resolver rotor 20 is 150, if the resolver rotor 20 rotates once, a signal having the above-mentioned phases A, B and C is obtained at a cycle of 150. The thus obtained 3-phase signal is phase converted by an electric circuit to a cos signal and a sin signal, the thus converted cos and sin signals are input into an RDC (resolver digital converter) and converted to a position signal, and the thus converted position signal is fed back to the DD motor, thereby being able to control the rotational speed and rotational angle (position) of the DD motor with accuracy.
In the thus structured conventional VR-type resolver, because the teeth number of the rotor is set as a large number such as 120, 150, 160 or the like in order to obtain a high resolving power, and also because the teeth top portion of the lamination of the resolver is formed in a square shape which is similar to the shape of the VR motor lamination, the smaller the diameter of the resolver is, the severer the working accuracy of the resolver is, so that a teeth pitch error and the like are easy to occur. Accumulation of the teeth pitch errors worsens the whole periphery accumulation accuracy of the rotation position detection, which gives rise to the error of the stationary angle of the resolver.
By the way, a test was conducted on the conventional VR-type resolver in which the teeth number of the resolver rotor 20 is 150 and the number of the magnetic poles 12 (which is hereinafter referred to as the pole number simply) is 18. In this test, the whole periphery (absolute) accuracy in one rotation of the rotor was measured using a rotary encoder and the measured data were recorded by an X-Y plotter. FIG. 24 shows the thus obtained whole periphery accuracy of the conventional VR-type resolver. The accuracy was 31.66 sec., that is, about 32 sec. As means for enhancing this accuracy further, it can be imagined that the pole number of the magnetic poles of the resolver stator arranged at regular intervals in the circumference thereof is increased. This ought to disperse and reduce influences by the teeth pitch errors of the resolver rotor, thereby being able to enchance the whole periphery accuracy further. Accordingly, with the two following conditions taken into account, let us check whether it is possible or not to increase the pole number (18 poles) of the resolver stator in the above-mentioned VR-type resolver.
(1) The pole number of a stator forming a 3-phase resolver must be a multiple of 3 and an even number. The reason why the pole number must be an even number is that the coils are wound in alternate directions around the respective magnetic poles of the stator, in particular, Cw (clockwise), CCW (counterclockwise), CW, CCW, - - - .
(2) The phases of the mutually adjoining magnetic poles of the resolver stator must be .+-.120.degree. (.+-.1/3 pitch).
When the teeth number of the resolver rotor is 150, the relation between the pole number (which is greater than or equal to 18) of the resolver stator and phase difference, which can satisfy the present condition, is shown in Table 1.
TABLE 1 __________________________________________________________________________ 18 24 30 36 42 48 54 60 66 72 78 84 90 120 90 0 60 206 45 280 180 98.2 30 332 283 240 __________________________________________________________________________ Upper : Stator pole number Lower : Phase difference
As can be seen clearly from Table 1, the stator pole numbers that can be set by the 3-phase resolver are only 18 and 90. However, if 90 magnetic poles are provided, then the width of the magnetic poles of the resolver stator is almost the same as the teeth width of the resolver rotor (the teeth width of 150 teeth is 0.787), with the result that the strength of the resolver stator is lowered excessively and thus the resolver stator can be damaged when it is manufactured. Therefore, it is difficult to realize the 90 magnetic poles. That is, in order to enhance further the detect accuracy of the VR-type resolver, it is desirable to increase the pole number of the resolver stator but, in fact, such increase in the pole number is limited by the teeth number of the resolver rotor, which makes it difficult to stabilize the high detect accuracy.